Ask A Scientist

As I ventured into the Education Development Center’s Genetic Counseling web lab this week one of the studies related to sickle-cell anemia. I was surprised to learn the allele for sickle cell anemia was considered recessive since it expresses itself. I decided this was a question I would pose to the scientist: Why is the allele for sickle cell anemia considered recessive if it expresses itself?

The response I received from the scientist was very informative. Before responding to my question he said how important it was to first understand how red blood cells carry oxygen to the entire body. Hemoglobin, a vital oxygen-carrying molecule consists of four protein subunits, two copies each of the alpha chain and the beta chain, which combine with iron to form the fully functional molecule. Hemoglobin binds oxygen in the lungs and then releases it in tissues and organs where it is needed.

When normal hemoglobin becomes deoxygenated, it remains soluble. When hemoglobin S molecules unload their oxygen cargo to a tissue, the deoxygenated hemoglobin S molecules bind to each other and crystallize within red blood cells by forming large sheet-like aggregates. This deforms the red blood cells, making them rigid and unable to bend as they travel through small blood vessels, so that they get stuck and trigger the characteristic—and painful—crises of sickle cell anemia.

It is due to these physical symptoms of the disease that sickle cell anemia is considered to be a recessive genetic condition. People who inherit only one sickle cell allele (and therefore have equal numbers of normal and mutant hemoglobin molecules in their red blood cells) normally don’t have the disease.

On the other hand, people who inherit two copies of the mutated gene (these people are called homozygotes) develop active sickle cell disease. Because two sickle cell alleles are required to engender the disease, sickle cell anemia is considered to be recessive and the sickle cell allele is called the recessive allele.

One item I found quite interesting is extreme conditions where environments have very low oxygen availability, such as very high altitudes, people with one sickle cell allele can develop symptoms of the disease. The sickle cell allele would be considered dominant under these circumstances.

I will begin implementing this website into my classroom this week. In my classroom there is a discussion board for math and science. While introducing content all through its completion, students generate excellent questions which are then placed on the discussion board. We usually research their questions on the Internet as a whole class. Due the experience afforded me today, I will have the students research their own questions via Ask a Scientist. Their questions and responses will then be posted to our class blog allowing all students and parents to glean knowledge.

References:

Ask a Scientist. (2010). Ask a question. Retrieved from http://www.askascientist.org/

Education Development Center. (2010). Genetics webs lab directory. Retrieved from http://www2.edc.org/weblabs/WebLabDirectory1.html